Otherwise eggs, quick to make and masses of varieties.
If you have a little more time or want low carb, one of my favorites to make is scrambled eggs + goat cheese, with turkey sausage. Lean protein, the goat cheese adds creaminess and flavor and the fat from the cheese can help you feel full longer (add the cheese after you remove from heat, or stir in just before you're ready to take the eggs off the stove). Personally I love Gordan Ramsay's scrambled eggs method – slow cooked over medium heat, so creamier/runnier eggs (depending on how long you cook them). This was a new thing I learned to make last year and it's becoming my favorite comfort food to make for myself (and also cheap!).